1. Field of the Invention
This invention relates to integrated-circuit (IC) chips including capacitor elements formed during the IC processing. More particularly, this invention relates to IC capacitor elements having improved performance characteristics, particularly a low variation in capacitance with change in applied voltage.
2. Description of the Prior Art
Integrated circuit chips commonly are made with capacitor elements which are integral parts of the circuitry on the chip. Conventionally, such capacitors are of the so-called "double poly" type, wherein both capacitor plates (upper and lower) are formed of polysilicon (i.e., silicon having a multiple crystalline structure rather than a monocrystalline structure). Capacitors also have been made wherein the upper surface of the lower polysilicon layer is clad with titanium silicide (TiSi.sub.2), which is a conductive material capable of serving as the lower plate of the capacitor. It also has been proposed to employ the IC metallization material (typically Aluminum) as the upper plate of the capacitor.
Prior art capacitors, although commercially useful, have not been fully satisfactory in certain respects. For example, double-poly capacitors generally have not had a sufficiently low voltage coefficient, which represents the amount of change in capacitance with change in applied voltage. Integrated circuits intended for high-performance applications, such as those involving high-resolution A/D and D/A converters, generally require a voltage coefficient less than 10 parts per million (ppm).